Variable reluctance motors (VR motors) are typically used as step motors because they can produce rotation in small, discrete steps. This mode of operation is inherent in the nature of VR motors. VR motors have a multi-pole stator and a multi-pole rotor, with the separation between poles on the stator, the pitch, different from that on the rotor. The stator poles are electromagnetically excited in separate groups or phases and the rotor rotates until its poles reach a position of minimum magnetic reluctance relative to the excited stator poles. Upon energizing successive stator phases, the rotor turns a distance equal to the rotor pitch minus the stator pitch.
Other characteristics of variable reluctance motors including their low cost, small size and high torque to inertia ratio make VR motors attractive for use as general purpose servomotors. Their brushless construction makes VR motors particularly suitable for applications requiring spark-free operation.
However, two drawbacks have limited the use of variable reluctance motors as servomotors: torque ripple and an nonlinear torque to input current ratio (T/I). Torque ripple is the variation in maximum available output torque as the position of the rotor poles varies with respect to the stator poles. The nonlinear T/I ratio is inherent in the design of typical VR motors because they have no permanent magnets. Torque is created by the interaction of two magnetic fields, the rotor field and the stator field, both a function of current.
Efforts to overcome the torque ripple problem have had only limited success. One approach produces constant torque by modulating the current to the motor, limiting the current during the high torque part of the cycle. This has the consequent disadvantage of also limiting the maximum torque developed by the motor to a level which can be as much as 70% below peak torque.
Another, more successful technique is to energize more than one phase during those portions in the motor's rotation where the torque from the individual phases is near its minimum. This reduces the torque ripple significantly, i.e., to about 80% of peak torque, but it also requires a more complex commutator to control the energization of the stator phases, and is less effective at high current levels.
In the past, in order to optimize the torque characteristics of VR motors, the stator has been the determinative element in designing the motor. Stator design balanced magnetic flux leakage, caused by having too many teeth too close together, against minimum holding torque at the stable detent position, caused by having too few teeth. The, stator was designed with many teeth of uniform cross section, to provide the maximum practical area at the tips of the teeth for the magnetic flux, while maintaining sufficient intertooth space for the winding coils. The ratio of the width of the stator teeth to the width of the gap between the teeth, called the stator tooth ratio, was typically 1.0 or more. The rotor design was dependent on the stator design, with the number and width of the rotor teeth chosen to suit the geometry of the stator. This resulted in a rotor tooth ratio of about 0.5.
It has been the practice to avoid operating VR motors at current levels that would cause the stator teeth to become magnetically saturated. With many uniform poles, the VR motor could run close to the saturation point, without entering saturation where an increase in current would produce only a negligible increase in torque.